Man is used to look at water flows in a very utilitarian manner, and the press (either generalist of scientific) often uses the expression "water resources" as a synonym to "river / groundwater flow". In this presentation, our aim is to discuss this unfortunate semantic shortcut, which causes much confusion in all discussions concerning water security issues. More specifically, we want to focus on seven issues of importance for a realistic assessment of water resources, which we will illustrate by actual examples: 1.the ratio of water resource to total water flow is a function of the hydrologic regime A water flow can only be accounted as a resource if it can be used by man, i.e. if it occurs at the right time. Naturally, what is the right time for flow depends on the type of use: for irrigation, the right time will be the summer. For hydroelectricity, the right time will be the peak hours of electrical consumption, etc. Since groundwater flows are much more regular than surface water flows, this notion mainly applies to river flow. It is interesting to note here that river floods should not be accounted as water resources, unless a river has enough dam storage capacity to regulate them. 2.the ratio of water resource to total water flow can be increased by construction of storage reservoirs. Man has the possibility to manipulate the natural variability of river flows, by building reservoirs to store water: they represent the only possibility to transform a larger part of river flow into a usable water resource. It is interesting to note here that because of the unavoidable evaporative losses caused by reservoir lakes, increasing the ratio of water resource to total water flow has a water cost. 3.the ratio of water resource to total water flow can be modified through aquifer management practices. Groundwater flow is usually much slower (and more regular than river flow)… but it still cannot be stopped. Aquifer management practices may in some extreme cases increase the usability of groundwater flow, by maintaining lower than natural groundwater levels in order to create storage space. This is mainly the case for karstic aquifers. 4.environmental constraints imply a reduction on available water resources Environmental law prescribes to maintain or reserve minimum river flows to protect river ecosystems. These environmental constraints (which are naturally beneficial to the ecosystems they are deemed to protect) reduce the availability to other users. 5.transboundary rivers and aquifers imply a reduction on available water resources (and incidentally, this means that international statistics overestimate world water resources).Rivers and aquifers do not respect state boundaries: they flow through them. Unless specific treaties have been signed to detail how transboundary flows are to be shared, each of the neighbour states usually counts the entire amount of transboundary flow as its own water resource. This is for example the case in FAO's AQUASTAT database. This means that international statistics overestimate world water flows… and thus world water resources. Even if water flow figures are right, water resources account can be wrong. 6.potential climatic changes hold the potential to reduce available water resources… even if they do not modify total water flows Future climatic changes may change or not precipitation amounts and river flows. Predictions are still quite uncertain. However, trends in temperature are extremely trustworthy, and they imply that in mountainous and northern regions, less snowfall will occur and more rainfall. This will have a definite impact on the seasonality of river flows, and potentially reduce the available water resources… even where total water flows are not modified. 7.the green water - blue water distinction must be handled with caution in water security computations, due to the fundamental asymmetry that exists between the two categories. Falkenmark proposed to distinguish "Green" water which corresponds to the part of precipitation stored in soil and subsequently transpired by vegetation from "Blue" water which corresponds to the part of precipitation that flows in rivers and aquifers. Although it was a tradition to only account for "Blue" water in water resources statistics, some authors have recently started to present figures for both "Blue" and "Green". We consider this to be potentially misleading: while it is easy to convert "Blue" water into "Green" water, the opposite is impossible. Rainfed agriculture (based on "Green" water) and irrigated agriculture (based on a mix of "Blue" and "Green" water) both contribute to food security. We would argue that water security issues are restricted to "Blue" water availability.